520 Astronomie und zugeordnete Wissenschaften
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Context
Thermonuclear supernovae (SNe), a subset of which are the highly important SNe Type Ia, remain one of the more poorly understood phenomena known to modern astrophysics.
In recent years, the single degenerate helium (He) donor channel, where a white dwarf star accretes He-rich matter from a hydrogen-depleted companion, has emerged as a promising candidate progenitor scenario for these events.
An unresolved question in this scenario is the fate of the companion star, which would be evident as a runaway hot subdwarf O/B stars (He sdO/B) in the aftermath of the SN event.
Aims
Previous studies have shown that the kinematic properties of an ejected companion provide an opportunity to closer examine the properties of an SN progenitor system. However, with the number of observed objects not matching predictions by theory, the viability of this mechanism is called into question.
In this study, we first synthesize a population of companion stars ejected by the aforementioned mechanism, taking into account predicted ejection velocities, the inferred population density in the Galactic mass distribution, and subsequent kinematics in the Galactic potential. We then discuss the astrometric properties of this population.
Methods
We present 10(6) individual ejection trajectories, which were numerically computed with a newly developed, lightweight simulation framework. Initial conditions were randomly generated, but weighted according to the Galactic mass density and ejection velocity data. We then discuss the bulk properties (Galactic distribution and observational parameters) of our sample.
Results
Our synthetic population reflects the Galactic mass distribution.
A peak in the density distribution for close objects is expected in the direction of the Galactic centre. Higher mass runaways should outnumber lower mass ones. If the entire considered mass range is realised, the radial velocity distribution should show a peak at 500 km s(-1).
If only close US 708 analogues are considered, there should be a peak at (similar to 750-850) km s(-1). In either case, US 708 should be a member of the high-velocity tail of the distribution.
Conclusions
We show that the puzzling lack of confirmed surviving companion stars of thermonuclear SNe, though possibly an observation-related selection effect, may indicate a selection against high mass donors in the SD He donor channel.
Solar H alpha excess during Solar Cycle 24 from full-disk filtergrams of the Chromospheric Telescope
(2022)
Context
The chromospheric H alpha spectral line is a strong line in the spectrum of the Sun and other stars. In the stellar regime, this spectral line is already used as a powerful tracer of stellar activity. For the Sun, other tracers, such as Ca II K, are typically used to monitor solar activity. Nonetheless, the Sun is observed constantly in H alpha with globally distributed ground-based full-disk imagers.
Aims
The aim of this study is to introduce the imaging H alpha excess and deficit as tracers of solar activity and compare them to other established indicators. Furthermore, we investigate whether the active region coverage fraction or the changing H alpha excess in the active regions dominates temporal variability in solar H alpha observations.
Methods
We used observations of full-disk H alpha filtergrams of the Chromospheric Telescope and morphological image processing techniques to extract the imaging H alpha excess and deficit, which were derived from the intensities above or below 10% of the median intensity in the filtergrams, respectively. These thresholds allowed us to filter for bright features (plage regions) and dark absorption features (filaments and sunspots). In addition, the thresholds were used to calculate the mean intensity I-mean(E/D) for H alpha excess and deficit regions. We describe the evolution of the H alpha excess and deficit during Solar Cycle 24 and compare it to the mean intensity and other well established tracers: the relative sunspot number, the F10.7 cm radio flux, and the Mg II index. In particular, we tried to determine how constant the H alpha excess and number density of H alpha excess regions are between solar maximum and minimum. The number of pixels above or below the intensity thresholds were used to calculate the area coverage fraction of H alpha excess and deficit regions on the Sun, which was compared to the imaging H alpha excess and deficit and the respective mean intensities averaged for the length of one Carrington rotation. In addition, we present the H alpha excess and mean intensity variation of selected active regions during their disk passage in comparison to the number of pixels of H alpha excess regions.
Results.
The H alpha excess and deficit follow the behavior of the solar activity over the course of the cycle. They both peak around solar maximum, whereby the peak of the H alpha deficit is shortly after the solar maximum. Nonetheless, the correlation of the monthly averages of the H alpha excess and deficit is high with a Spearman correlation of rho = 0.91. The H alpha excess is closely correlated to the chromospheric Mg II index with a correlation of 0.95. The highest correlation of the H alpha deficit is found with the F10.7 cm radio flux, with a correlation of 0.89, due to their peaks after the solar activity maximum. Furthermore, the H alpha deficit reflects the cyclic behavior of polar crown filaments and their disappearance shortly before the solar maximum. We investigated the mean intensity distribution for H alpha excess regions for solar minimum and maximum. The shape of the distributions for solar minimum and maximum is very similar, but with different amplitudes. Furthermore, we found that the area coverage fraction of H alpha excess regions and the H alpha excess are strongly correlated with an overall Spearman correlation of 0.92. The correlation between the H alpha excess and the mean intensity of H alpha excess regions is 0.75. The correlation of the area coverage fraction and the mean intensity of H alpha excess regions is in general relatively low (rho = 0.45) and only for few active regions is this correlation above 0.7. The weak correlation between the area coverage fraction and mean intensity leaves us pessimistic that the degeneracy between these two quantities can be broken for the modeling of unresolved stellar surfaces.
We study the first-arrival (first-hitting) dynamics and efficiency of a one-dimensional random search model performing asymmetric Levy flights by leveraging the Fokker-Planck equation with a delta-sink and an asymmetric space-fractional derivative operator with stable index alpha and asymmetry (skewness) parameter beta.
We find exact analytical results for the probability density of first-arrival times and the search efficiency, and we analyse their behaviour within the limits of short and long times.
We find that when the starting point of the searcher is to the right of the target, random search by Brownian motion is more efficient than Levy flights with beta <= 0 (with a rightward bias) for short initial distances, while for beta>0 (with a leftward bias) Levy flights with alpha -> 1 are more efficient.
When increasing the initial distance of the searcher to the target, Levy flight search (except for alpha=1 with beta=0) is more efficient than the Brownian search. Moreover, the asymmetry in jumps leads to essentially higher efficiency of the Levy search compared to symmetric Levy flights at both short and long distances, and the effect is more pronounced for stable indices alpha close to unity.
EC 22536-5304
(2021)
Helium-burning hot subdwarf stars of spectral types O and B (sdO/B) are thought to be produced through various types of binary interactions. The helium-rich hot subdwarf star EC 22536-5304 was recently found to be extremely enriched in lead. Here, we show that EC 22536-5304 is a binary star with a metal-poor subdwarf F-type (sdF) companion. We performed a detailed analysis of high-resolution SALT/HRS and VLT/UVES spectra, deriving metal abundances for the hot subdwarf, as well as atmospheric parameters for both components. Because we consider the contribution of the sdF star, the derived lead abundance for the sdOB, + 6.3 +/- 0.3 dex relative to solar, is even higher than previously thought. We derive T-eff = 6210 +/- 70 K, log g = 4.64 +/- 0.10, [FE/H] = - 1.95 +/- 0.04, and [alpha/Fe] = + 0.40 +/- 0.04 for the sdF component. Radial velocity variations, although poorly sampled at present, indicate that the binary system has a long orbital period of about 457 days. This suggests that the system was likely formed through stable Roche lobe overflow (RLOF). A kinematic analysis shows that EC 22536-5304 is on an eccentric orbit around the Galactic centre. This, as well as the low metallicity and strong alpha enhancement of the sdF-type companion, indicate that EC 22536-5304 is part of the Galactic halo or metal-weak thick disc. As the first long-period hot subdwarf binary at [FE/H] less than or similar to- 1, EC 22536-5304 may help to constrain the RLOF mechanism for mass transfer from low-mass, low-metallicity red giant branch (RGB) stars to main-sequence companions.
Thermal electrons have gyroradii many orders of magnitude smaller than the finite width of a shock, thus need to be pre-accelerated before they can cross it and be accelerated by diffusive shock acceleration. One region where pre-acceleration may occur is the inner foreshock, which upstream electrons must pass through before any potential downstream crossing. In this paper, we perform a large-scale particle-in-cell simulation that generates a single shock with parameters motivated from supernova remnants. Within the foreshock, reflected electrons excite the oblique whistler instability and produce electromagnetic whistler waves, which comove with the upstream flow and as nonlinear structures eventually reach radii of up to 5 ion-gyroradii. We show that the inner electromagnetic configuration of the whistlers evolves into complex nonlinear structures bound by a strong magnetic field around four times the upstream value. Although these nonlinear structures do not in general interact with cospatial upstream electrons, they resonate with electrons that have been reflected at the shock. We show that they can scatter, or even trap, reflected electrons, confining around 0.8% of the total upstream electron population to the region close to the shock where they can undergo substantial pre-acceleration. This acceleration process is similar to, yet approximately three times more efficient than, stochastic shock drift acceleration.
Assessing the impact of hydrogen absorption on the characteristics of the Galactic center excess
(2022)
We present a new reconstruction of the distribution of atomic hydrogen in the inner Galaxy that is based on explicit radiation transport modeling of line and continuum emission and a gas-flow model in the barred Galaxy that provides distance resolution for lines of sight toward the Galactic center.
The main benefits of the new gas model are (a) the ability to reproduce the negative line signals seen with the HI4PI survey and (b) the accounting for gas that primarily manifests itself through absorption.
We apply the new model of Galactic atomic hydrogen to an analysis of the diffuse gamma-ray emission from the inner Galaxy, for which an excess at a few GeV was reported that may be related to dark matter.
We find with high significance an improved fit to the diffuse gamma-ray emission observed with the Fermi-LAT, if our new H i model is used to estimate the cosmic-ray induced diffuse gamma-ray emission.
The fit still requires a nuclear bulge at high significance. Once this is included there is no evidence of a dark-matter signal, be it cuspy or cored. But an additional so-called boxy bulge is still favored by the data.
This finding is robust under the variation of various parameters, for example, the excitation temperature of atomic hydrogen, and a number of tests for systematic issues.
Gravitational waves from the collision of binary neutron stars provide a unique opportunity to study the behaviour of supranuclear matter, the fundamental properties of gravity and the cosmic history of our Universe. However, given the complexity of Einstein's field equations, theoretical models that enable source-property inference suffer from systematic uncertainties due to simplifying assumptions. We develop a hypermodel approach to compare and measure the uncertainty of gravitational-wave approximants. Using state-of-the-art models, we apply this new technique to the binary neutron star observations GW170817 and GW190425 and to the sub-threshold candidate GW200311_103121. Our analysis reveals subtle systematic differences (with Bayesian odds of similar to 2) between waveform models. A frequency-dependence study suggests that this may be due to the treatment of the tidal sector. This new technique provides a proving ground for model development and a means to identify waveform systematics in future observing runs where detector improvements will increase the number and clarity of binary neutron star collisions we observe.
The current paradigm of cosmic-ray (CR) origin states that the greater part of galactic CRs is produced by supernova remnants. The interaction of supernova ejecta with the interstellar medium after a supernova's explosions results in shocks responsible for CR acceleration via diffusive shock acceleration (DSA). We use particle-in-cell (PIC) simulations and a combined PIC-magnetohydrodynamic (PIC-MHD) technique to investigate whether DSA can occur in oblique high Mach number shocks. Using the PIC method, we follow the formation of the shock and determine the fraction of the particles that gets involved in DSA. With this result, we use PIC-MHD simulations to model the large-scale structure of the plasma and the magnetic field surrounding the shock and find out whether or not the reflected particles can generate upstream turbulence and trigger DSA. We find that the feasibility of this process in oblique shocks depends strongly on the Alfvenic Mach number, and the DSA process is more likely to be triggered at high Mach number shocks.
Most image restoration methods in astronomy rely upon probabilistic tools that infer the best solution for a deconvolution problem. They achieve good performances when the point spread function (PSF) is spatially invariant in the image plane.
However, this condition is not always satisfied in real optical systems. We propose a new method for the restoration of images affected by static and anisotropic aberrations using Deep Neural Networks that can be directly applied to sky images.
The network is trained using simulated sky images corresponding to the T80-S Telescope optical model, a 80-cm survey imager at Cerro Tololo (Chile), which are synthesized using a Zernike polynomial representation of the optical system.
Once trained, the network can be used directly on sky images, outputting a corrected version of the image that has a constant and known PSF across its field of view. The method is to be tested on the T80-S Telescope.
We present the method and results on synthetic data.
3A 1954+319 has been classified for a long time as a symbiotic X-ray binary, hosting a slowly rotating neutron star and an aged M red giant. Recently, this classification has been revised thanks to the discovery that the donor star is an M supergiant. This makes 3A 1954+319 a rare type of high-mass X-ray binary consisting of a neutron star and a red supergiant donor. In this paper, we analyse two archival and still unpublished XMM-Newton and NuSTAR observations of the source. We perform a detailed hardness ratio-resolved spectral analysis to search for spectral variability that could help investigating the structures of the inhomogeneous M supergiant wind from which the neutron star is accreting. We discuss our results in the context of wind-fed supergiant X-ray binaries and show that the newest findings on 3A 1954+319 reinforce the hypothesis that the neutron star in this system is endowed with a magnetar-like magnetic field strength (greater than or similar to 10(14) G).